Can't change screen background/wallpaper

On my desktop machine running 16.04LTS I can't change the background/wallpaper - it's stuck on 'Warty-final-ubuntu.png' and won't change. Right-clicking on an image & selecting 'Set as wallpaper' produces no discernible effect, certainly doesn't change the wallpaper! and neither do wallpaper-changers like Variety & Wallch. My laptop doesn't have this problem, has anyone any suggestions as to how to get the wallpaper changing to work as the purple screen is getting boring!